The present invention is directed generally to electrically conductive polymer compositions and electrical devices comprised thereof. More particularly, the invention is directed to electrically conductive thermoset polymer compositions that contain substantially discrete distributed thermoplastic polymers and electrically conductive particles and exhibit positive temperature coefficients for use in current limiting devices and other electrical applications.
Conductive polymer compositions are finding increasingly widespread use in electrical applications because conductive polymer have desirable properties, such as the solid state characteristics and positive temperature coefficient (PTC) behavior. Generally, a material is said to have a positive temperature coefficient if the electrical resistance of a material increases with temperature.
Certain PTC materials exhibit an exponential rise in the resistivity of the material above a given temperature. Conductive polymers that exhibit this type of PTC behavior have been used in current limiting circuit protection, circuit breaker, applications for a variety of electrical devices, such as motors, solenoids, phone lines, and batteries. When a short circuit or a fault occurs causing a rise in the line current, IR resistive heating will cause a temperature rise in the polymer. The sudden rise in the polymer temperature will trigger a sudden increase in the polymer resistivity that will in effect "trip the circuit" by limiting the flow of current. Conductive polymer circuit breakers are advantageous because, when the excess voltage and current diminishes, the conductive polymer will again have a lower resistivity upon cooling. Therefore, unlike manual circuit breakers and fuses, the conductive polymer circuit breakers do not have to be reset or replaced each time the circuit is tripped.
Conductive polymers that exhibit exponential PTC behavior have been traditionally made by dispersing a conductive material in a crystalline polymer. In crystalline conductive polymers, the temperature at which the polymer switches from a low resistivity to a high resistivity, called the switching temperature, T.sub.s, is associated with the breakdown of the crystalline structure near the melting point of the polymer. A disadvantage of crystalline conductive polymers is that the switching temperature is governed by the physical properties of the polymer and can not be made specific to the desired application.
Alternatively, amorphous and semi-crystalline polymers, such as elastomers, thermoplastics and thermosets have been crosslinked to produce polymers that exhibit PTC behavior analogous to the crystalline polymer compositions. Crosslinking is usually accomplished by chemical treatment or by irradiation of the polymer. The temperature at which the crosslinking was performed determines the switching temperature of the resultant conductive polymer composition. Conductive polymers in which PTC behavior is produced by crosslinking have increased application as a result of the ability to control the switching temperature of the conductive polymer.
However, elastomers and thermoplastics typically have limited thermal and electrical resistance stability when exposed to thermal cycling, such as in current limiting applications. Whereas, thermoset polymer devices provide increased thermal stability, but the thermoset polymers having a high conductive material loading generally do not exhibit PTC effects that are necessary for current limiting applications.
Additionally, blended polymer compositions have been used to produce conductive polymer compositions that have PTC characteristics. For example, U.S. Pat. No. 5,250,228 (Baigrie et. al.) discloses compositions that have conductive material dispersed in a mutually soluble mixture of an essentially amorphous thermoplastic and a liquid thermoset polymer. The miscible polymer mixtures provide for increased conductive material loadings compared to prior thermoset polymer compositions, combined with increased thermal stability over compositions utilizing only thermoplastic polymers.
A disadvantage of these materials are that the polymers used in the material are limited to miscible polymers. Also, while the addition of a thermoset to the matrix material provides some degree of increased stability compared to a pure, essentially amorphous thermoplastic matrix material, the material remains relatively unsuitable for high temperature and current limiting applications. In addition, the low temperature resistance of these materials tends to increase with successive thermal cycles, known as "ratcheting", unless strict processing controls are followed.
Thus, it is apparent that the conductive polymer compositions currently available do not provide the versatility and durability required to become a viable alternative to mechanical circuit breakers in the industry. Accordingly, there is a need for conductive polymer compositions that are thermally stable and can be processed to have PTC characteristics in both high and low resistance devices and applications in a cost effective manner.